Shira Scheindlin, a U.S. District Court judge for the Southern District of New York, has ruled that New York City's "stop and frisk" policy violates the Fourteenth Amendment's promise of equal protection, as black and Hispanic people are subject to stops and searches at a higher rate than whites. Mayor Michael Bloomberg responded by deriding Scheindlin for not acknowledging the policy's benefits, noting that "nowhere in her 195 page decision does she mention the historic cuts in crime or the number of lives that have been saved."

But what, exactly, does "stop and frisk" entail? Is it racially biased? Does it actually reduce crime? Here's what you need to know.

What is stop and frisk?

"Stop, question and frisk" is an NYPD policy wherein police will detain and question pedestrians, and potentially search them, if they have a "reasonable suspicion" that the pedestrian in question "committed, is committing, or is about to commit a felony or a Penal Law misdemeanor."

How many stops are conducted? Who gets stopped?

According to a report from the Public Advocate's office, 532,911 stops were conducted in 2012, down from 685,724 in 2011. The vast majority of those stops were of black or Hispanic people:

And the pace is increasing, as this chart by Jeffrey Fagan at Columbia Law School shows:

According to the New York Civil Liberties Union, 97,296 stops were conducted in 2002. That's less than a fifth of the number of stops conducted in 2012.

The racial breakdown in 2012 in keeping with patterns over the past decade, according to this chart from Adam Serwer and Jaeah Lee at Mother Jones:

Note that the number of stops does not capture how many individual people are stopped, as many individuals are stopped multiple times.

Where are people stopped?

The precincts doing the most stops tend to be in Brooklyn — particularly East New York, Starret City, Brownsville and Ocean Hill, but also Bed-Stuy, Bushwick and Flatbush — and the Bronx, with a few in Staten Island, Jamaica in Queens and Harlem thrown in for good measure. By contrast, the areas with the least stops tend to be ones with lots of white people: Midtown, Little Italy, Chelsea and Central Park in Manhattan, and Greenpoint in Brooklyn.

What accounts for why there are more stops in some areas than in others?

A man is stopped and frisked by NYPD in Brooklyn, New York. (Photo by Jeremy Sparig)

It depends whom you ask. The Bloomberg administration says that it's focusing stops on areas with lots of crime. But Fagan found that even if you control for the crime rate, the racial makeup of a precinct is a good predictor of the number of stops.

"The percent Black population and the percent Hispanic population predict higher numbers of stops, controlling for the local crime rate and the social and economic characteristics of the precinct," Fagan's report explains. "The crime rate is significant as well, so the identification of the race effects suggests that racial composition has a marginal influence on stops, over and above the unique contributions of crime." That finding holds up both in earlier years — such as 1998 and 1999, which Fagan analyzed with Andrew Gelman and Alex Kiss — as well the time period since Fagan's initial report came out in 2010.

Tracey Meares, a Yale law professor, explains that if the NYPD were doing what it claims, then a scatterplot with the number of stops on the Y axis and the crime rate on the X axis would show a linear relationship -- meaning that stops would straightforwardly increase along with the crime rate. That doesn't happen. "What you see is that that relationship is curvilinear and it's concave, so the police districts in the middle get a lot more stops than you'd think that they should be getting based on the crime rate," Meares says. That suggests some racial bias in the implementation of stop and frisk.

How many stops result in arrests or tickets?

Not a whole lot. Serwer and Lee have another chart:

Wow, that looks super-biased on the part of the NYPD. But its's not the only study.

An anti-stop and frisk protest. (Seth Wenig / AP)

The NYPD commissioned a study by the RAND Corp. — in particular Greg Ridgeway, acting director of the National Institute for Justice (the Department of Justice's research arm) — which concluded that "black pedestrians were stopped at a rate that is 20 to 30 percent lower than their representation in crime-suspect descriptions. Hispanic pedestrians were stopped disproportionately more, by 5 to 10 percent, than their representation among crime-suspect descriptions would predict." Ridgeway also found that the NYPD "frisked white suspects slightly less frequently than they did similarly situated nonwhites" and that "black suspects are slightly more likely to have been frisked than white suspects stopped in circumstances similar to the black suspects."

However, Fagan has levied a fairly devastating set of objections to Ridgeway's methodology. Among other issues, the RAND study tries to match up stops to compare how whites and blacks are treated but in doing so fails to account for basic things like which potential crime prompted the stop and how reasonable the cop's suspicion was. The sample of officers the RAND study looks at isn't representative, and the benchmark they use to determine the races of those stopped is derived from analysis of violent crimes, which make up a tiny fraction of stops. Fagan concludes that "the analyses in the report are unreliable and methodologically flawed to the extent that it is not reliable evidence that racial bias is absent in NYPD stop and frisk activity."

Does it reduce crime?

New York City Police chief Ray Kelly (right) has been a champion of stop and frisk. (Seth Wenig / AP)

"Anyone who says we know this is bringing the crime rate down is really making it up," Fagan says. Others wouldn't put it that harshly, but the evidence does seem to suggest that stop and frisk is, at best, ineffective, and, at worst, actively alienates communities with whom the police need to engage.

There have been three studies to date evaluating the effectiveness of stop and frisk. The first, an unpublished paper by NYU's Dennis Smith and SUNY Albany's Robert Purtell, found "statistically significant and negative effects of the lagged stop rates on rates of robbery, burglary, motor vehicle theft, and homicide and no significant effects on rates of assault, rape, or grand larceny," according to a summary here. "They also found evidence of 'declining returns to scale' (i.e., diminishing effects over time) of the effects of police stops on most of the offenses they analyzed but increasing returns to scale for robbery."

The second (free copy here), by University of Missouri-St Louis's Richard Rosenfeld and Arizona State's Robert Fornango, throws cold water on even Smith and Purtell's modest positive findings on robbery and burglary. They find the stops "show few significant effects of several SQF [stop, question, and frisk] measures on precinct robbery and burglary rates."

The third, by Hebrew University's David Weisburd and George Mason's Cody Telep and Brian Lawton, analyzes where stop and frisk incidents occur to determine whether the program counts as "hot spots" policing, a strategy with demonstrable effectiveness wherein police target resources in geographic areas with heavy crime. The researchers find that the pattern of stops is consistent with a hot spots approach. But this says nothing about the effectiveness of this particular type of hot spots policing . "Given the possible negative impacts of SQF policing, both on citizens who live in such areas, and the primarily young and minority population that is the main subject of SQFs, we suspect especially in the long run that this approach will lead to unintended negative consequences," the authors write.

That much is obvious: Stop and frisk is alienating the communities it targets. It's done so since the late 1990s, when stop and frisk incidents ratcheted considerably and culminated in the death of Amadou Diallo, an innocent 22-year-old West African immigrant who was shot 41 times by NYPD officers as part of a stop. That spurred an investigation by the New York attorney general's office, then headed by Eliot Spitzer, into that policing program. Such incidents have real costs. Fagan, Meares, and NYU's Tom Tyler note that there's a huge research literature showing that perceptions of police legitimacy matter for crime rates, and we know that invasions of privacy like stops and searches, particularly when conducted rudely, damage police legitimacy.

Are there other possible explanations for the crime drop?

This is the real kicker. As Kevin Drum says in Mother Jones, the thing driving the drop in crime in New York, as everywhere, might not have anything to do with policing. It's likely the removal of lead from gasoline and house paint, he argues. Several studies have found that lead exposure can damage children's brain development, affecting their behavior. Rick Nevin, and economist and a leading researcher on crime and lead questions, notes that there has been far more progress on removing lead in New York City than in other large cities like Chicago or Detroit:

New York's lead removal efforts are commendable and are a more than adequate explanation of why it's seen sharper crime drops than other cities. There's no reason to credit alienating policies like stop and frisk here.

What now?

Judge Scheindlin has named Peter Zimroth, a former lawyer for the City of New York now at Arnold & Porter, to oversee the NYPD. She also mandated a number of other remedies, including a requirement that some police officers wear cameras, changes to training and disciplinary policies, and a process to devise broader reforms to stop and frisk that involves "representatives of religious, advocacy, and grassroots organizations; NYPD personnel andrepresentatives of police organizations; the District Attorneys’ offices…the Mayor’s office, the NYPD, and the lawyers in this case; and the non-parties that submitted briefs: the Civil Rights Division of the DOJ, Communities United for Police Reform, and the Black, Latino, and Asian Caucus of the New York City Council."

The city will almost certainly appeal, and a higher court could issue a stay on Scheindlin's ruling, but for the time being it's the binding policy on stop and frisk.

Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.

To pause and restart automatic updates, click "Live" or "Paused". If paused, you'll be notified of the number of additional comments that have come in.

Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.